I Few Weeks Ago A Big Bruise Randomly Appeared On

I few weeks ago a big Bruise randomly appeared on my back it looked as if I was bleeding under the skin, I didn't notice it because I never did anything, I never fell or was hit on the back it came out of nowhere, it doesn't hurt but yesterday the same thing randomly came back in the same spot my aunt pointed it out because it looked as if I had a very large cut on my back and again it is not hurting and it looked like I was bleeding under the skin in that one spot but I never did anything to make it appear..... Any help